Blue Monkeys Shift Feeding and Movement Between Forest Zones in Ethiopia

That is the central finding of a fifteen-month study published in BMC Zoology, which compared the daily activity budgets of two habituated groups of Cercopithecus mitis boutourlinii living in different zones of Saja Forest, part of the Kafa Biosphere Reserve. The research, conducted from August 2024 to October 2025, recorded 19,725 behavioural observations from 4,675 instantaneous group scans across 126 observation days, offering the first detailed look at how these Vulnerable, Ethiopia-endemic monkeys allocate their time across contrasting forest environments.

The study’s overall picture is one of a foraging-dominated primate. Across both groups, feeding accounted for the largest share of the activity budget at 29.6 percent, while a residual category of other behaviours, including urination, defecation, vocalisation and drinking, made up the smallest at 7.5 percent. But the aggregate figure conceals a sharp divide between the two study groups. The core-zone group, numbering between 37 and 41 individuals, devoted significantly more time to feeding than its transition-zone counterpart, with monthly feeding budgets ranging from 30.26 to 36.50 percent compared with 24.07 to 27.71 percent for the transition group. The difference was statistically robust under Mann–Whitney U testing (U = 22.0, p < 0.001).

The transition-zone group, smaller at 21 to 23 individuals, compensated with significantly more locomotion. Moving occupied 28.06 to 30.83 percent of its monthly budget against 23.62 to 26.50 percent for the core-zone group, a difference the authors also report as significant (U = 39.0, p = 0.002). Resting, by contrast, did not differ significantly between the groups (U = 98.0, p = 0.548), and neither did socialising (U = 70.0, p = 0.078) or other activities (U = 87.5, p = 0.300). Social activity was numerically higher in the transition-zone group at 13.5 percent, but the authors are careful to note this difference fell short of statistical significance.

A two-way multivariate analysis of variance, run on monthly behavioural counts with Pillai’s Trace as the test statistic, confirmed that both study group and season shaped the activity budgets independently. Study group produced a significant multivariate effect (Pillai’s Trace = 0.746, F(5, 22) = 12.939, p < 0.001), as did season (Pillai’s Trace = 0.538, F(5, 22) = 5.118, p = 0.003). Crucially, the interaction between the two was not significant (Pillai’s Trace = 0.117, F(5, 22) = 0.584, p = 0.712), meaning that both groups adjusted their behaviour to seasonal change in broadly similar ways, with the between-group differences persisting throughout the year rather than appearing or disappearing with the rains.

Season itself left a clear imprint on the monkeys’ daily schedules. Movement ran in the opposite direction, peaking in the wet season at 28.67 percent against 25.84 percent in the dry season. The authors note that this wet-season movement pattern differs from many primate studies that report greater travel during periods of food scarcity, and they caution that because food availability was never directly measured, the underlying drivers of these seasonal shifts cannot be determined from the present data. Resting, socialising and other activities remained statistically stable across seasons.

Within a single day, the monkeys followed a bimodal rhythm familiar from other diurnal primates. Feeding peaked in the early morning and again in the late afternoon, while resting reached its maximum around midday. Movement stayed relatively constant across the day. A chi-square test confirmed a highly significant association between activity type and time interval (χ² = 27,872, df = 16, p < 0.001), though the authors again stress that environmental variables such as food distribution were not measured, so the mechanisms behind the daily pattern remain open questions.

Sex and age also shaped behaviour, though modestly. Adult females devoted the greatest proportion of their time to feeding at 33.3 percent, juveniles socialised more than any other class at 19.3 percent, and sub-adult males and females moved more (28.2 and 27.9 percent respectively) than adult males at 24.8 percent. Adult males rested the most at 27.1 percent, while juveniles rested the least at 17.9 percent. The overall association between activity and sex–age category was statistically significant (Pearson’s χ² = 356.81, df = 16, p < 0.001), but the effect size was very small (Cramer’s V = 0.08), indicating that sex and age explain only a modest fraction of the total behavioural variation.

Monthly comparisons, meanwhile, revealed remarkable temporal stability. Although feeding remained the dominant activity in every month and resting and moving fluctuated moderately, Kruskal–Wallis tests found no statistically significant monthly differences for any behaviour: feeding (H = 10.72, df = 14, p = 0.708), resting (H = 13.46, df = 14, p = 0.491), moving (H = 8.43, df = 14, p = 0.866), socialising (H = 13.70, df = 14, p = 0.472) or other activities (H = 10.35, df = 14, p = 0.736). The monkeys, it seems, adjusted fine-grained time allocation without ever abandoning their overall behavioural strategy.

The authors are unusually explicit about the limits of their design, and this candour matters for how the findings should be read. Only one habituated group was observed per habitat type, so habitat was never independently replicated; every difference between the zones could equally reflect group-specific traditions, ranging patterns or the substantial size disparity between the two groups. Habitat characteristics such as vegetation structure, canopy cover and resource availability were not directly measured, and reproductive status and dominance relationships were not evaluated. The researchers therefore frame their results as comparisons between two study groups rather than general habitat-level effects, a caution they repeat throughout the discussion.

Even with those caveats, the study delivers the first baseline activity-budget data for Boutourlini’s blue monkey in the Kafa Biosphere Reserve, where previous work had documented only the subspecies’ distribution. Saja Forest, roughly 1,968 hectares crossed by a main road and surrounded by subsistence farming, coffee and honey production, hosts five primate species alongside buffalo, lion, leopard and more than 250 bird species. For a highly arboreal monkey dependent on intact canopy, itself classified as Vulnerable and threatened by ongoing forest degradation, understanding how time budgets shift across zones and seasons is a practical starting point for conservation planning. The authors recommend that future work include multiple independent groups per habitat type, groups of comparable size, and direct measurements of food availability and nutritional quality, so that habitat effects can finally be separated from the idiosyncrasies of individual monkey groups.

Subject of Research: Activity budget variation between two Boutourlini's blue monkey groups in different forest zones of Saja Forest, Ethiopia

Article Title: Habitat-related variation in daily activity patterns of Boutourlini’s blue monkey (Cercopithecus mitis boutourlinii) in Saja Forest, Kafa Biosphere Reserve, Southwest Ethiopia

Article References: Negesse, M., Gadisa, T., & Habtamu, T. (2026). Habitat-related variation in daily activity patterns of Boutourlini’s blue monkey (Cercopithecus mitis boutourlinii) in Saja Forest, Kafa Biosphere Reserve, Southwest Ethiopia. BMC Zoology, 11(1), Article 24. https://doi.org/10.1186/s40850-026-00280-4

Image Credits: AI Generated

DOI: 10.1186/s40850-026-00280-4

Keywords: Boutourlini's blue monkey, Cercopithecus mitis boutourlinii, activity budget, Kafa Biosphere Reserve, Ethiopia, primate behaviour, seasonal variation, instantaneous scan sampling, habitat variation, behavioural flexibility, Saja Forest, conservation